1. Field of the Invention
This invention relates to an oil-free vacuum pump comprising a two-stage integral scroll pump for evacuating a vessel.
2. Description of Related Art
In a pump of the stationary/revolving scroll type, a revolving scroll is revolved about a stationary scroll without being rotated, thus varying the volume of a closed space formed between two laps.
The revolving scroll is caused to undergo revolution with a fixed radius about the center of the lap of the stationary scroll such that the point of contact between the two laps defining the closed space noted above, which functions as a compression chamber, is gradually shifted toward the center of the system. Gas, which is withdrawn from an intake, is led around the winding end of the second lap to enter the closed space between the two laps. With the revolution of the revolving scroll, the withdrawn gas is pressurized as it is shifted toward the system center while its volume is reduced and, when the closed space is connected with a discharge port, the gas is exhausted to the outside.
Nowadays, along with a demand for vacuum degree increase, a reduction of the time of operation until a desired vacuum degree is obtained is demanded.
Low compression ratio vacuum pumps require considerable time for evacuation, and therefore high compression ratio vacuum pumps are desired.
The high compression ratio can be increased by increasing the turn number of the spiral scrolls. Increasing the turn number of a scroll, however, increases the outer size of the scroll, thus giving rise to problems such as shaft vibration due to sagging at high rotational speeds, and also generation of noise, heat and wear caused by non-uniform contact between the stationary and revolving scrolls.
To solve these problems, it is conceivable to use two vacuum pumps, which have small scroll turn numbers and thus small scroll sizes, and to drive these pumps by coupling the intake port of the second stage pump to the discharge port of the first one.
When this method of driving (i.e. coupling or connecting) is adopted, however, in an initial stage of pumping, the pressure in the sealed vessel connected to the system is close to the atmospheric pressure. A high pressure is built up in the inter-scroll space due to the high compression ratio, resulting in the generation of high heat. In this case, it is necessary to cause the compressed gas under high pressure to escape to the outside.
As a related technique, Japanese Laid-Open Patent Publication No. 62-48979 discloses a structure for reducing the pump load at the start of the pump. Specifically, when the pressure in a first space defined by a stationary scroll and a revolving scroll becomes higher than the pressure in the next space, the gas in the first space is exhausted through a valve into the second space, so that the gas is exhausted to the outside when the second space is vented through a discharge port or vent connected with the outside.
In this technique, a vent is provided in a central part of a polished member of the stationary scroll, and a valve chamber is provided near the discharge port. The valve chamber is connected with a first connection hole, which is open to a first closed space or gas pocket defined by stationary and revolving scrolls and is led from the end of the revolving scrawl into the first gas pocket. The valve chamber is also connected with a second connection hole, which is formed near the discharge port and is open to a second closed space or gas pocket defined by stationary and revolving scrolls during compression of gas before compressed gas is exhausted to the outside and also when compressed gas is exhausted from the discharge port to the outside. A valve is provided in the opening of the first connection hole in the valve chamber. In this structure, when the pressure in the first gas pocket becomes higher than that in the second gas pocket, the valve is opened to cause the gas in the first gas pocket to be exhausted into the second gas pocket.
It is conceivable to apply this technique to driving two small scroll size, small scroll turn number vacuum pumps by coupling the intake of the second stage pump to the discharge port of the first stage pump. In this case, the valve may be provided on the first stage pump, so that an increase of the pressure in the first gas pocket beyond a predetermined level causes the first connection hole to be opened by the valve to exhaust the compressed gas in the first gas pocket into the second gas pocket.
With the revolution of the revolving scroll, however, the second gas pocket is connected with the discharge port, which is connected with the intake of the second stage pump.
Consequently, gas that has been compressed in the first stage pump is entirely led to the second stage pump. Therefore, like the first stage pump, high pressure is also built up in the second stage pump gas pocket defined by the stationary and revolving scrolls, thus resulting in high heat generation.